(Coefficient of Thermal Expansion) The rate at which a material changes dimension as its temperature is increased. Can refer to the linear dimensions or the volume of the material. Expressed as the fractional change in dimension divided by the change in temperature, most commonly as ppm/°C. Often referred to as thermal coefficient of expansion (TCE). Note particularly that: CTE is not a linear function of temperature, it being quite common for materials to expand and shrink differently at different temperatures. For example, water first shrinks and then expands as it cools, having a maximum density at around 4°C. CTE is not necessarily isotropic, that is, the same in all directions. Most metals are isotropic, but most laminate materials have a CTE in the Z axis (through the thickness of the laminate) which is different from that of the XY axis (in the same plane as the laminate). Stresses from the higher Z-axis expansion have a major impact on the reliability of through-hole metallisation.